Abstract class for representing access to a system resource.
All permissions have a name (whose interpretation depends on the subclass),
as well as abstract functions for defining the semantics of the
particular Permission subclass.

Most Permission objects also include an "actions" list that tells the actions
that are permitted for the object. For example,
for a java.io.FilePermission object, the permission name is
the pathname of a file (or directory), and the actions list
(such as "read, write") specifies which actions are granted for the
specified file (or for files in the specified directory).
The actions list is optional for Permission objects, such as
java.lang.RuntimePermission,
that don't need such a list; you either have the named permission (such
as "system.exit") or you don't.

An important method that must be implemented by each subclass is
the implies method to compare Permissions. Basically,
"permission p1 implies permission p2" means that
if one is granted permission p1, one is naturally granted permission p2.
Thus, this is not an equality test, but rather more of a
subset test.

Permission objects are similar to String objects in that they
are immutable once they have been created. Subclasses should not
provide methods that can change the state of a permission
once it has been created.

Method Detail

checkGuard

Implements the guard interface for a permission. The
SecurityManager.checkPermission method is called,
passing this permission object as the permission to check.
Returns silently if access is granted. Otherwise, throws
a SecurityException.

hashCode

public abstract int hashCode()

Returns the hash code value for this Permission object.

The required hashCode behavior for Permission Objects is
the following:

Whenever it is invoked on the same Permission object more than
once during an execution of a Java application, the
hashCode method
must consistently return the same integer. This integer need not
remain consistent from one execution of an application to another
execution of the same application.

If two Permission objects are equal according to the
equals
method, then calling the hashCode method on each of the
two Permission objects must produce the same integer result.

getActions

Returns the actions as a String. This is abstract
so subclasses can defer creating a String representation until
one is needed. Subclasses should always return actions in what they
consider to be their
canonical form. For example, two FilePermission objects created via
the following:

newPermissionCollection

Returns an empty PermissionCollection for a given Permission object, or null if
one is not defined. Subclasses of class Permission should
override this if they need to store their permissions in a particular
PermissionCollection object in order to provide the correct semantics
when the PermissionCollection.implies method is called.
If null is returned,
then the caller of this method is free to store permissions of this
type in any PermissionCollection they choose (one that uses a Hashtable,
one that uses a Vector, etc).

Returns:

a new PermissionCollection object for this type of Permission, or
null if one is not defined.

toString

Returns a string describing this Permission. The convention is to
specify the class name, the permission name, and the actions in
the following format: '("ClassName" "name" "actions")', or
'("ClassName" "name")' if actions list is null or empty.